Process for producing a planar body of an oxide single crystal

ABSTRACT

A planar body with a good crystallinity is grown continuously and stably when a planar body of an oxide single crystal is grown by a micro pulling-down method. A raw material of the oxide single crystal is melted in a crucible  7 . A planar seed crystal  15  is contacted to a melt  18 , and then the melt  18  is pulled down from an opening  13   c  of the crucible  7  by lowering the seed crystal. A planar body  14  is produced following the seed crystal  15 . In this case, differences in lattice constants between each crystal axis of the seed crystal  15  and each corresponding crystal axis of the planar body  14  is controlled at 0.1% or less, respectively.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a process for producing a planar body ofan oxide single crystal.

[0003] 2. Description of the Related Art

[0004] A single crystal of lithium potassium niobate and a singlecrystal of lithium potassium niobate-lithium potassium tantalate solidsolution have been noted especially as single crystals for blue lightsecond harmonic generation (SHG) device for a semiconductor laser. Thedevice can emit even the ultraviolet lights having the wavelengths of390 nm or so, thus the crystals can be suitable for wide applicationssuch as optical disk memory, medicine and photochemical fields, andvarious optical measurements by using such short-wavelength lights.Since the above single crystals have a large electro-optic effect, theycan be also applied to optical memory devices using theirphoto-refractive effect.

[0005] However, for an application of a second harmonic generationdevice, for example, even a small fluctuation in a composition of thesingle crystal may affect the wavelength of the second harmonic wavegenerated by the device. Therefore, the specification of the range ofthe composition required for said single crystals is severe, and thefluctuation in the composition should be suppressed in a narrow range.However, since the composition consists of as many as three of fourcomponents, growing a single crystal at a high rate is generallyextremely difficult to achieve, while controlling the proportion of thecomponents to be constant.

[0006] In addition, for optical applications, especially for anapplication for the second harmonic wave generation, a laser beam havinga short wavelength of, for example, about 400 nm needs to propagate inthe single crystal at as a high power density as possible. Moreover, thephoto deterioration has to be controlled to the minimum at the sametime. In this way, since controlling the photo deterioration isessential, the single crystal has to possess a good crystallinity forthis purpose.

[0007] Moreover, lithium niobate and lithium potassium niobate can besubstituted between cations, thus solid solution in which the cationsare solid-solved is produced. Therefore, controlling the composition ofthe melt needs to grow a single crystal of a specific composition. Promsuch a background, a double crucible method and a method of growing acrystal while feeding raw materials have been examined mainly for the CZmethod and the TSSG method. For example, Kitamura et al. tried to grow alithium niobate single crystal of a stoichiometric composition bycombining an automatic powder feeder to a double crucible CZ method (J.Crystal Growth, 116 (1992), p.327). However, it was difficult to enhancea crystal growth rate with these methods.

[0008] NGK Insulators, Ltd. suggested a micro pulling-down method forgrowing the above single crystal with a constant compositionalproportions, for example, in JP-A-8-319191. In this method, a rawmaterial, for example, comprising lithium potassium niobate is put intoa platinum crucible and melted, and then the melt is pulled downgradually and continuously through a nozzle attached to the bottom ofthe crucible. The micro pulling-down method can grow a single crystalmore rapidly than the CZ method or the TSSG method does. Moreover, thecompositions of the melt and the grown single crystal can be controlledby growing the single crystal continuously with feeding the rawmaterials for growing the single crystal to the raw material meltingcrucible.

SUMMARY OF THE INVENTION

[0009] However, there is still a limitation in using a micropulling-down method to grow a good single crystal plate (a planar bodyof a single crystal) continuously at a high rate. Because, when theplanar body of the single crystal is pulled down with the seed crystalplate, cracks tend to occur near an interface boundary between the seedcrystal and the planar body.

[0010] It is an object of the invention to prevent cracks occurring nearthe interface boundary between a seed crystal and a planar body and togrow a planar body of an oxide single crystal having a goodcrystallinity continuously and stably, when the planar body of the oxidesingle crystal is grown with the micro pulling-down method.

[0011] The inventors had examined various methods to grow planar bodiesof oxide single crystals, which use the micro pulling-down method. As aresult, the inventors found that cracks were likely to occur when adifference in lattice constant between the seed crystal and the planarbody was large. Thus, the planar body having a good crystallinity may becontinuously made by contacting a planar seed crystal to a melt, pullingdown the melt from an opening of a crucible by lowering the seedcrystal, forming a planar body following the seed crystal, andcontrolling difference in lattice constant between each of crystal axesof the seed crystal and corresponding crystal axes of the planar body at0.1% or less, respectively.

[0012] In this case, the lattice constant of each crystal axis of theplanar body can be adjusted by controlling the proportions of therespective components in the crucible. Taking lithium potassium niobate,for example, the lattice constant of each crystal axis in the grownplanar body can be changed by slightly changing a relative ratio ofniobium, lithium and potassium in the crucible.

[0013] Preferably, the differences between the lattice constants of theplanar body and the respective ones of the planar seed crystal arefurther reduced as the width of the planar body become large.Specifically, when the width of the planar body is 30-50 mm, it is morepreferable to control the differences between the corresponding latticeconstants at 0.06% or less. When the width of the planar body is 50 mmor more, it is more preferable to control the differences of the latticeconstants at 0.04% or less.

[0014]FIG. 1 is a schematic sectional view of a manufacturing apparatusfor growing a single crystal. FIGS. 2(a) and (b) represent steps ofpulling down a planar body of the single crystal.

[0015] A crucible 7 is placed in a furnace body. An upper furnace unitis arranged to surround the crucible 7 and an upper space 5 thereof, andhas a heater 2 buried therein. A nozzle 13 extends downwardly from abottom part of the crucible 7. The nozzle 13 comprises a connecting-tubeportion 13 a and a planar expanded portion 13 b at the lower end of theconnecting-tube portion 13 a. In FIG. 1, only a cross sectional view ofthe planar expanded portion 13 b is shown. The connecting-tube portion13 a and the planar expanded portion 13 b can be changed variously inshape. Both 13 a and 13 b can also be arbitrarily changed incombination. A slender opening 13 c is formed at the lower end of theplanar expanded portion 13 b, and a vicinity of the opening 13 c is asingle crystal-growing portion 19. A lower furnace unit 3 is arranged tosurround the nozzle 13 and a surrounding space 6 thereof, and has aheater 4 buried therein. The crucible 7 and the nozzle 13 are bothformed from a corrosion-resistant conductive material.

[0016] One electrode of a power source 10 is connected to a point A ofthe crucible 7 with an electric cable 9, and the other electrode of thepower source 10 is connected to a lower bent B of the crucible 7. Oneelectrode of an another power source 10 is connected to a point C of theconnecting-tube portion 13 a with an electric cable 9, and the otherelectrode of the power source 10 is connected to a lower end D of theplanar expanded portion 13 b. These current-carrying systems areisolated from each other and configured to control their voltagesindependently.

[0017] An after-heater 12 is located in the space 6 to surround thenozzle 13 with a distance. An intake tube 11 extends upwardly in thecrucible 7 and an intake opening 22 is provided at the upper end of theintake tube 11. The intake opening 22 slightly protrudes from a bottomportion of a melt 8.

[0018] The upper furnace unit 1, the lower furnace unit 3 and theafter-heater 12 are allowed to heat for setting an appropriatetemperature distribution in each of the space 5 and space 6. Then a rawmaterial for the melt is supplied into the crucible 7 and theelectricity is supplied to the crucible 7 and the nozzle 13 for heating.In this condition, the melt slightly protrudes from the opening 13 c atthe single crystal-growing portion 19.

[0019] In this condition, a planar seed crystal 15 is held with a holder21 at both side faces and moved upwardly as shown in FIG. 2(a), and theupper surface of the seed crystal 15 is contacted with the meltprotruding from the opening 13 c. At that time, a uniform solidphase-liquid phase interface (meniscus) is formed between the upper endof the seed crystal 15 and the melt 18 pulled downwardly from the nozzle13. Then, the seed crystal 15 is lowered as shown in FIG. 2(b). As aresult, a planar body 14 is continuously formed on an upper side of theseed crystal 15 and pulled downwardly.

[0020] The lattice constant is measured by an X-ray diffractionapparatus (MRD diffractometer, manufactured by Philips). If anunequivalent crystal axis exists in an oxide single crystal, differencesin lattice constants between each crystal axis of the seed crystal andeach corresponding axis of the planar body, respectively, have to be0.1% or less.

[0021] An oxide single crystal is not particularly limited, but, forexample, lithium potassium niobate (KLN), lithium potassiumniobate-lithium potassium tantalate solid solution (KLTN:[K₃Li_(2-x)(Ta_(y)Nb_(1-y))_(5+x)O_(15+2x)]) lithium niobate, lithiumtantalate, litium niobate-lithium tantalate solid solution,Ba_(1-x)Sr_(x)Nb₂O₆, Mn—Zn ferrite, yttrium aluminum garnet substitutedwith Nd, Er and/or Yb, YAG, and YVO₄ substituted with Nd, Er, and/or Ybcan be exemplified.

EXAMPLE 1

[0022] With a single crystal-producing apparatus shown in FIG. 1, aplanar body of a lithium potassium niobate single crystal was producedaccording to the invention. Specifically, the temperature of the wholefurnace was controlled by the upper furnace unit 1 and the lower furnaceunit 3. The apparatus was configured to be able to control thetemperature gradient near the single crystal-growing portion 19 by anelectric supply to the nozzle 13 and the heat generation of theafter-heater 12. A mechanism of pulling down the single crystal platewas equipped, in which a single crystal plate was pulled down withcontrolling the pulling-down rate uniformly within a range from 2 to 100mm/hour in a vertical direction.

[0023] A planar seed crystal 15 of lithium potassium niobate was used. Asize of the seed crystal 15 was 30 mm×1 mm in cross-section and 5 mm inlength. Lattice constants of the seed crystal 15 were 12.568 Å in a-axisand 4.031 Å in c-axis. The molar ratio of potassium, lithium and niobiumwas 30: 18.2:51.8. A half width of an X-ray rocking curve was 60 secondsat 0 0 4 reflection of the seed crystal (measured by the MRDdiffractometer, manufactured by Philips).

[0024] Potassium carbonate, lithium carbonate and niobium pentoxide wereprepared at a molar ratio of 30:26:44 to produce a raw material powder.

[0025] The raw material was supplied into the platinum crucible 7, andthe crucible 7 was set in place. With controlling the temperature of thespace 5 in the upper furnace unit 1 within a range from 1100 to 1200°C., the raw material in the crucible 7 was melted. The temperature ofthe space 6 in the lower furnace unit 3 was controlled uniformly withina range from 500 to 1000° C. While a given electric power was suppliedto each of the crucible 7, the nozzle 13 and the after-heater 12, asingle crystal was grown. In this case, the temperature of the singlecrystal growing portion could be at 980-1150° C., and the temperaturegradient of the single crystal growing portion could be controlled at10-150° C./nm.

[0026] The crucible 7 had an elliptical cross-sectional shape, whereinthe major axis, the minor axis and the height was 50 mm, 10 mm and 10mm, respectively. The length of the connecting-tube portion was 5 mm. Across-sectional dimension of the planar expanded portion 13 b was 1mm×50 mm. A dimension of the opening 13 c was 1 mm long×50 mm wide.Under such conditions, the seed crystal 15 was pulled down at a rate of10 mm/hour.

[0027] While the raw material in equal weight to that of thecrystallized melt was being fed to the crucible 7, the crystal was keptgrowing, and the planar body 14 was cut off from the nozzle 13 andcooled when the length of the planar body 14 reached 50 mm. The latticeconstant of the obtained planar body was measured to give the a-axislength of 12.569 Å and the c-axis length of 4.029 Å. A molar ratio ofpotassium, lithium and niobium was 30:18.1:51.9, respectively. Adifference between the lattice constant of the planar seed crystal 15and that of the planar body 14 (lattice mismatch) was 0.01% or less ina-axis and 0.05% in c-axis. However, no crack occurred at the joiningportion between the planar seed crystal 15 and the planar body 14. Also,a half width of an X-ray rocking curve was 40 seconds at 0 0 4reflection of the planar body.

EXAMPLE 2

[0028] Similar results as Example 1 were obtained with a plate oflithium potassium niobate-lithium potassium tantalate solid solutionsingle crystal.

EXAMPLE 3

[0029] A planar body of lithium niobate was grown according to Example 1except that a planar seed crystal 15 of lithium niobate was used.Dimensions of the seed crystal 15 were 50 mm×1 mm in cross-section and 5mm in length. The seed crystal was cut out and obtained from astoichiometric lithium niobate single crystal grown by the Czochralskimethod, wherein a direction of pulling down the seed crystal was setparallel to the X-axis, and a direction of a growing face was setparallel to the Z-axis. A lattice constant of the seed crystal was 5.148Å in a-axis and 13.857 Å in c-axis. A molar ratio of lithium and niobiumwas 50:50, respectively. A half width of an X-ray rocking curve was 12seconds at 0 0 12 reflection of the seed crystal.

[0030] Lithium carbonate and niobium pentoxide were prepared at themolar ratio of 58:42 to produce a raw material powder. The raw materialwas fed into the platinum crucible 7, and the crucible 7 was arranged ina predetermined place. While a temperature of the space 5 in the upperfurnace unit 1 was controlled within a range from 1200 to 1300° C., theraw material in the crucible 7 was melted. A temperature of the space 6in the lower furnace unit 3 was controlled uniformly within a range from500 to 1000° C. While a predetermined electric power was supplied toeach of the crucible 7, the nozzle 13 and the after-heater 12, a singlecrystal was grown. In this case, the temperature of the singlecrystal-growing portion could be at 1200-1250° C., and the temperaturegradient of the single crystal growing portion could be controlled at10-150° C./mm. The seed crystal was lowered at a rate of 30 mm/hour. Avolume of the crystallized lithium niobate was measured to convert it toa weight at each unit time internally, and the raw material of lithiumniobate in equal weight to the converted one was supplied into thecrucible. In this case, unlike the raw material powder melted at thebeginning, the lithium niobate powder supplied afterward in such mannerwas prepared to have a molar ratio of lithium and niobium at 50:50.

[0031] As a result, a planar body having the width of 50 mm was formedwithout a shoulder portion. While the raw material powder of lithiumniobate was fed, the crystal was continuously grown until a length ofthe planar body reached 50 mm. Then the planar body was cut off from thenozzle 13 and cooled.

[0032] A composition of the obtained planar body (Z-plate) was measuredby an inductively coupled plasma method, and a molar ratio of lithiumand niobium was 50:50, which corresponded to the stoichiometrycomposition. A lattice constant of the planar body was measured to givean a-axis length of 5.148 Å and the c-axis length of 13.858 Å.Differences of lattice constants between the planar body and the seedcrystal (lattice mismatch) were 0.01% or less in a-axis and 0.01% orless in c-axis. No cracks occurred at the joining portion between theseed crystal 15 and the planar body 14. A half width of an X-ray rockingcurve was 12 seconds at the planar body.

EXAMPLE 4

[0033] A planar body (X-plate) was grown according to Example 3 exceptthat the seed crystal was worked so that a direction of pulling down theplanar seed crystal might be parallel to the Z-axis, and that adirection of a growing face might be parallel to the X-axis. An a-axislength, a c-axis length and a half width of X-ray rocking curve at theseed crystal and the shoulder portion, were similar to those in Example3. No cracks also occurred at the joining face between the shoulderportion and the seed crystal.

[0034] As mentioned above, according to the invention, when the planarbody of the oxide single crystal was grown by the micro pulling-downmethod, cracks can be prevented near an interface between the seedcrystal and the planar body, and the planar body with good crystallinitycan be grown continuously and stably.

What is claimed is:
 1. A process for producing a planar body of an oxidesingle crystal, said process comprising the steps of melting a rawmaterial of said oxide single crystal in a crucible, contacting a planarseed crystal to the melt, pulling down said melt from an opening of saidcrucible by lowering the planar seed crystal, and forming said planarbody following said planar seed crystal, wherein differences in latticeconstants between each crystal axis of said planar seed crystal and eachcorresponding crystal axis of said planar body is 0.1% or less,respectively.
 2. A process for producing a planar body according toclaim 1 , wherein said difference between each lattice constant of eachcrystal axis of said planar seed crystal and each corresponding latticeconstant of each corresponding crystal axis of said planar body isadjusted by controlling a proportion of each component in the crucible.